Wear layer surfaces of vinyl flooring products are usually prepared from plastisols, which are dispersions of fine particles of resins in a plasticizer. Most plastisols for vinyl flooring products are formulated using polyvinyl chloride (PVC) emulsion resins mixed with primary and secondary plasticizers, extenders, stabilizers and other additives. After being applied to a substrate, for example a felt substrate or a glass fleece, by spread coating, the layer of plastisol is fused by heating to an elevated temperature. Wear layer surfaces of vinyl flooring products can also be prepared with a clear or colored powdered dry blend as described in French patent FR 2,542,260 published Sep. 14, 1984. These dry blend powders are prepared from polyvinyl chloride (PVC) suspension resins, plasticizers, stabilizers, extenders and other additives. They are prepared in a Henschel type mixer. The dry blend powder is applied to a felt or vinyl substrate and is fused by heating to an elevated temperature.
Clear wear layer surfaces can also be prepared by mixing polyvinyl chloride (PVC) suspension resins, plasticizers, stabilizers, extenders and other additives in a Banbury mixer and calendering or extruding the product to various gauges. This calendered sheet is then laminated to any one of various substrates, for example a PVC based film, a felt, a glass fleece, or the like.
Thermoplastic vinyl wear layers made from polyvinyl chloride (PVC) plastisols or from powdered dry blends or a high shear mixer (e.g. Banbury mixer) show various limitations and disadvantages, such as insufficient gloss retention, insufficient wear and abrasion resistance, stain resistance, scuff resistance, tear resistance, gauge resistance and resistance to various chemical agents. Some of these problems are additionally aggravated by migration of plasticizers towards the surface of the wear layer.
In order to improve wear properties of thermoplastic vinyl layers, various modifications to plastisol, dry blend and high shear mixer compositions have been proposed, mostly by changing the amount ad type of plasticizers. Changing the amount and type of external plasticizers has not produced any substantial improvements. External plasticizers conventionally used for PVC compositions include dialkyl phthalates, for example dioctyl phthalate. These conventional plasticizers have proven unsatisfactory for various reasons, one of which is that they have a tendency to migrate to the surface and exude from the surface. This leads to undesirable characteristics, including staining. It is also known to plasticize PVC internally. This is done, for instance, by copolymerizing a small amount of ethylene or propylene with the vinyl chloride, so that there is obtained PVC containing a small amount of ethylene or propylene incorporated into the polymer backbone. The small amount of ethylene or propylene may be up to about 10% of the total weight of the polyvinyl chloride (PVC) polymer. Internal plasticization has proved to be unsatisfactory in view of many limitations as to applications, formulations and performance. Plastisols having a resin component other than polyvinyl chloride (PVC) have also been proposed (see, for example, U.S. Pat. No. 4,210,567, U.S. Pat. No. 4,309,331 and U.S. Pat. No. 4,380,606), but this has not eliminated all the above mentioned problems.
To obtain a surface with acceptable wear properties, a thin top coat layer, usually made of a polyurethane, has been added over the plasticized polyvinyl chloride (PVC) coating (see, for example, U.S. Pat. No. 3,935,330, U.S. Pat. No. 4,100,318, U.S. Pat. No. 4,216,187, U.S. Pat. No. 4,217,396, U.S. Pat. No. 4,393,187 and U.S. Pat. No. 4,598,009). Although showing usually satisfactory mechanical resistance, these top coatings are not themselves free of problems.
The additional top coating and the process by which it is applied may adversely affect properties of an underlying foamable plastisol layer by damaging foam cells and causing reduction in the level of embossing. Also, since the polyurethane layer is expensive, it is usually thin. In some cases it may be too thin to prevent the migration of plasticizers from the base layer to the surface of the coating. Such migration may be prevented by increasing the thickness of the polyurethane top layer, but this makes the flooring material more expensive.
Most top layers of polyurethane are prepared by covering the polymer base layer, which may be formed for example from a polyvinyl chloride (PVC) plastisol, a polyolefin, a polyester, a polyamide, a polyepoxy or a polyacrylate, with a liquid composition of monomers (U.V. curable) or polymers (solvent or water based) which composition is subsequently cured at an elevated temperature or U.V. cured to produce a crosslinked, thermoset, mechanically resistant polyurethane coating. The liquid composition of monomers or polymers has a low viscosity which decreases with increasing temperature, before the top layer becomes solid by crosslinking. As a result, the top coating of polyurethane is usually of an uneven thickness, due to the low viscosity monomer or polymer composition flowing into any embossed valleys. Another disadvantage, with a foamable material, is that the hot melt viscosity of the polyurethane composition is too low to prevent the escape of gases from the underlying foamed or foaming plastisol layer. This results in blistering and pin holes of the polyurethane layer. To prevent this from happening, in the past a layer of a composition having a high melt viscosity has been situated between the foamed or foaming plastisol layer and the polyurethane composition. A suitable composition having a high melt viscosity that has been used for this purpose is a plasticized high temperature melt viscosity PVC resin.
FR 2,379,323, published Sep. 1, 1978, discloses a top coat composition that contains, additionally to the usual polyurethane polymers, an ethylenically unsaturated compound. After being applied to a foamable substrate but prior to being thermally cured, the composition is irradiated with U.V. light or an ionizing radiation or is heated to a low temperature by infrared radiation. This causes polymerization of the ethylenically unsaturated compound, which increases the hot melt viscosity of the composition and changes it to a solid state. The partially crosslinked composition creates a high temperature melt flow barrier film that prevents the escape of gases from the foamed underlayer when the latter is fused at a high temperature. The ethylenically unsaturated compounds used for this purpose are all very expensive monomers.
A similar two-step coating process is disclosed in U.S. Pat. No. 3,935,330. A coating composition, which comprises both thermally and radiation-curable components, is first partially cured by exposing it to an ionizing or non-ionizing radiation and the cure is then completed by a thermal treatment. Ionizing radiation is a radiation produced by an electron beam or electron generating sources. Non-ionizing radiation is a radiation produced by Carbon ARC, Tungsten filament lamps, sunlamps, lasers, Mercury arc, Xenon arcs or any other source of ultra violet and visible light radiation. Including thermally curable components into the coating composition and adding the step of thermal curing improves properties of the coating compared with coatings cured by radiation only. U.S. Pat. No. 3,935,330 is clearly concerned with coatings to be applied to wood and metal, and this method of processing is not practical for foaming products.